The present invention relates to an optical sensor, an image forming device, and a method for correcting a toner concentration.
A conventional image forming device includes, a fixing device, a color concentration sensor (also called a toner concentration sensor) such as a reflection type optical sensor with a light emitting element, and a light receiving element so that a concentration of toner formed on an intermediate transfer belt is detected. Specifically, the light emitting element irradiates an area with light causing the toner of the intermediate transfer belt to adhere to the area, and the light receiving element receives reflected light of the irradiated light to output a detection voltage in accordance with a light receiving quantity.
However, in the detection voltage of the color concentration sensor, there is a noise component caused by, for example, a dark current by electrons and holes generated thermally in addition to electrons and holes generated by photoelectric conversion by light, an offset voltage of an amplifier circuit, and a minute bias current flowing in or flowing out with respect to an input terminal of the amplifier circuit. The noise poses a problem since the color concentration sensor cannot precisely sense toner concentration (also called color concentration). Thus, in order to sense the precise toner concentration, it is necessary to correct a noise component. When the noise component is a +voltage, the noise component can be sensed even if the color concentration sensor is driven by a single power supply, and thus, the correction is possible. However, when the noise component is a −voltage, substantially 0 V is outputted if the color concentration sensor is driven by the single power source, so that the noise component cannot be sensed. Thus, the correction is impossible. A technique for solving this problem is disclosed, for example, in Japanese Unexamined Patent Publication No. 2009-003283. In Japanese Unexamined Patent Publication No. 2009-003283, there has been disclosed a method in which the color concentration sensor is driven by a dual power supply, and the noise components of both the +voltage and the −voltage can be sensed, which reduces influence of the offset voltage.
However, as to the technique described in Japanese Unexamined Patent Publication No. 2009-003283, the image forming device needs to be provided with a part to adjust the offset voltage (a configuration to drive an operational amplifier by the dual power supply), which increases the number of parts in the image forming device. One technique for solving this problem is a technique of biasing an output value of the color concentration sensor disclosed in, for example, Japanese Unexamined Patent Publication No. H10-186827 is applied to set the offset voltage to a +voltage. This configuration reduces the influence of the offset voltage even if the amplifier circuit is driven by the single power supply instead of the dual power supply. However, there is a problem in that a dynamic range of the sensor becomes small.